This invention relates to a method for improving the properties of synthetic yellow iron oxide. More particularly the present invention relates to a method for easily improving synthetic yellow iron oxide, a compound which has been being consumed in large quantities, in terms of crystallinity, particle size distribution and particle shape and for imparting thereto heightened thermal resistivity and clear color tone.
The yellow pigments which are available today include lead chromate, strontium chromate, cadmium sulfide, benzidine yellow, etc. However, since these pigments are invariably noxious or carcinogenic substances, use of these pigments will be increasingly more restricted in the future with a view to preserving human health and preventing the environment from pollution. In view of the liklihood of such restriction, industries which produce or use coloring materials anxiously await the development of an excellent innoxious yellow pigment capable of taking the place of such noxious yellow pigments as mentioned above.
Unlike the yellow pigments mentioned above, yellow iron oxide is perfectly innoxious and also inexpensive and, therefore, has long been used in large quantities as the raw material for colorants and magnetic recording powders. Particularly because of its perfect freedom from noxiousness, it is used in imparting a color to the outer covering cigaret filters, to the feed used in poultry farming and to cosmetic materials. However, such yellow iron oxide still suffers from the disadvantage that the color tone produced thereby is not clear and the pigment itself has inferior thermal resistivity. For this reason, it has failed to find favorable acceptance as a substitute for said noxious yellow pigments.
At present, yellow iron oxide is wholly produced by resorting to a method whereby microcrystalline nuclei of ferrous sulfate (II) are caused to grow by the hydrolysis of the aqueous solution of ferrous sulfate (such as the method of Nobuoka et al as reported in Bulletin of the Government Industrial Research Institute, Osaka, 6, 135 (1955), or the method of Nobuoka et al as reported in Industrial Chemistry, 66, 412 (1963) etc.). The product by these methods is composed chiefly of .alpha.-FeOOH and has a structure of Goethite, with the component particles formed in the shape of needles resembling tactoids. These methods cause crystals to attain growth in the aqueous solution of a salt of sparing solubility nearing perfect insolubility. In view of this principle of production, it is substantially inconceivable that a product having a uniform particle size and possessing homogenous quality throughout can be obtained by merely adjusting the production conditions. Accordingly, some of the grades of synthetic yellow iron oxide available today on the market contain imperfectly crystallized particles and other grades contain large and small particles intermingled with each other.
It is already well known that the various properties exhibited by a powdery system generally are correlated with the morphology of the constituent particles of the powdery system. In the case of pigments, the morphology of particles has an effect upon such factors as color tone, hiding power, oil-absorbing property, tinting strength, rheological properties in paints, and film strength. In the case of yellow iron oxide, if large particles and small particles coexist therein in a mixed state, then the properties exhibited by the large particles and those exhibited by the small particles differ and, in this compound which exists in the form of a powder as a whole, they will offset each other and give rise to statistically averaged properties. In terms of color tone, for instance, the color produced by large particles and that produced by small particles differ from each other (Nobuoka et al: J. Chem. Soc. Japan, Ind. Chem. Sec., 66, 412 (1963)). When these particles are mixed, their respective colors intermingle to produce a diminished color in much the same way as when painting colors are mixed, with the result that both luminosity and chromaticity are decreased to produce a darkish color tone as a whole. Thus it is desirable to obtained a pigment in such form that the constituent particles thereof have as uniform a particle size as possible. The same principle also applies substantially to the effect of the presence of imperfect crystals upon the properties of a powdery matter.
Yellow iron oxide is used in large quantities as the raw material for magnetic recording media. For this purpose, .alpha.-FeOOH is subjected to a heat treatment for reduction and oxidation to produce the needle-shaped .gamma.-Fe.sub.2 O.sub.3. The morphological magnetic anisotropy and the acicular orientation of the product are utilized. In this case, since the heat treatment governs the morphology of magnetic particles of .gamma.-Fe.sub.2 O.sub.3 to be formed, the particle size distribution of the raw material and that of the magnetic powder as the product are closely related to each other. In the magnetic properties of a magnetic powder, particle size and the ratio between particle dimensions manifest themselves directly in the form of morphological magnetic anisotropy. To be more specific, the magnetic properties possessed by magnetic particles of different diameters are different from one another. The particle size distribution in a magnetic powder is accurately measured in terms of anisotropic magnetic field distribution and, thus, is utilized as the date for evaluation of acceptability for recording. If particles finer than the allowable lower limit are present, for example, they impair the transferring property and degrade the magnetic field orientation. Particles larger than the allowable upper limit are not desirable, for they give rise to a structure of multiple magnetic domains and bring about a decline of coercive force. The magnetic recording powder, therefore, proves suitable when it is in a form such that the range of particle size distribution is narrow and the acicular particles are as uniform as possible. In the light of the aforementioned principle of production, it is essential for the formation of such magnetic powder that the raw material .alpha.-FeOOH to be used should possess a uniform particle size distribution.
An object of the present invention is to provide a method for improving the properties of synthetic yellow iron oxide for thereby allowing imperfect crystals present in said compound to be converted into perfect crystals, the particle size distribution thereof to be substantially uniformized and the thermal resistivity thereof to be enhanced.